1. Field of the Invention
The present invention relates to an expanding mandrel, that is to say to a mandrel designed to grip a workpiece to be machined (turning, grinding) by pressing an expanding portion of the mandrel against the walls of an axial bore provided in said workpiece.
2. Description of the Related Art
There is known a first type of expanding mandrel, the body (xe2x80x9cmandrel shaftxe2x80x9d) of which comprises a forward portion having, prior to gripping, the shape of a hollow cylinder provided with several longitudinal slits. This split portion is caused to expand by sliding a countersunk head screw along its axis, so that the split portion assumes, during gripping, a conical shape the apex angle of which progressively increases. Said countersunk head screw is located at the end of a longitudinal tie member. When said mandrel is mounted on the machine spindle, a rod is screwed onto the tie member that can be caused to slide along the axis of the machine spindle to control the gripping or releasing of the workpiece. A locking nut can be used to secure the shaft of the mandrel to the forward face of the machine spindle.
Although this type of mandrel is relatively simple to manufacture, and inexpensive, it has a number of drawbacks. Firstly, conical expansion of the split portion is irregular, with the result that gripping lacks precision. Secondly, the split portion lacks rigidity and is liable to be deformed, or even fail, when the gripping mechanism is actuated in the presence or, which is worse, in the absence of a workpiece; in this case, the only way of replacing the defective part is to replace the entire mandrel. Similarly, the gripping diameter characterizing each mandrel is unique, to within one finishing operation, with the result that the user has to change mandrels when he wishes to machine a workpiece having a bore with a diameter that differs sufficiently from that of the previous workpiece. Such drawbacks are obviously time consuming and lead to considerable extra cost.
A second type of conventional mandrel (manufactured, moreover, by the Applicant of the present invention) is free of certain of these drawbacks. In this type of mandrel, the shaft has a forward part of a conical shape. Unlike the mandrel described above, it is the expanding portion in this case that will slide axially to effect gripping or releasing. This expanding portion is constituted by a ring provided with longitudinal slits (xe2x80x9cthe sleevexe2x80x9d), the inner wall of this sleeve having the shape of a cone, or taper, the apex angle of which is equal to that of the cone over which the sleeve slides; the outer surface of the sleeve thus remains rigorously cylindrical during its expansion, thus ensuring that the workpiece is gripped perfectly precisely and uniformly. In addition, there is no longer any risk of failure or plastic deformation of the sleeve when the control tie member is actuated, in the presence or in the absence of a workpiece. Finally, to change over from one workpiece to another having a different bore diameter, it suffices to replace the sleeve, and not the entire mandrel.
In this known mandrel, the sleeve extends rearwards via a coupling collar which is attached to the control tie member by means of a pin held in place by a ring encircling said collar. It takes some time to remove the sleeve from its coupling collar and to replace it with another, analogous sleeve. It has thus proved desirable, in order to be able to make the best possible use of the fact that this type of mandrel allows the expanding portion to be removed, to devise a system for fixing the sleeve to the mandrel that enables the sleeve to be changed quickly and easily.
The object of the present invention is thus to shorten considerably the time required to change sleeves, while, at the same time, retaining the advantages of this second type of conventional mandrel. For this purpose, it provides an expanding mandrel comprising a shaft having a conical forward end, an expanding sleeve constituted by a ring provided with longitudinal slits designed to slide over said conical forward end, the inner wall of the sleeve itself also having the shape of a cone the apex angle of which is equal to that of said conical forward end, and a tie member designed to be connected to a rod that can be caused to slide along the axis of the machine spindle to control the gripping or the release of a workpiece. According to the invention, said sleeve is connected to said tie member via a control ring integral with the tie member, and the mandrel comprises a removable quick mounting means enabling the sleeve to be secured to said control ring.
According to certain particular characteristics, the sleeve comprises radial projections, and said removable quick mounting means is constituted by a nut capable of being fixed on said control ring so as to trap the sleeve against the control ring by means of said radial projections.